PlotAfter a heated argument with Sheldon, Leonard hides out at Penny's apartment. He then goes into a flashback explaining his first meeting with Sheldon, how his group of friends first formed, and how the elevator got broken.
- Answer Cut: Penny makes fun of Leonard for almost blowing himself up seven years ago. Leonard counters by mentioning that she probably did something just as stupid back then. Cut to Penny seven years earlier, cheering that she got a negative pregnancy test.
- Call-Forward: The origin of many of the gags that have appeared throughout the series — the roommate agreement, the broken elevator, Sheldon's way of knocking, the couch and Sheldon's spot on it — are shown in this episode.
- Coitus Interruptus: Sheldon barges into Leonard's room the first time he brings a girl over.note Sheldon: (Knock, Knock, Knock) Leonard (Knock, Knock, Knock) Leonard (Knock, Knock, Knock) Leonard.
Leonard: What do you want? (Sheldon opens the door) I didn't say "Come in"!
Sheldon: You asked what I wanted. I wanted to come in.
- Defacement Insult: When Sheldon shows Leonard his room, it has "DIE, SHELDON, DIE!" painted on the wall in blood red letters. Provides the current page image.
- Explosive Stupidity: Leonard makes some rocket fuel for Howard's model rocket, ignoring Sheldon's warnings that the formulation is all wrong. When the fuel is about to explode, Sheldon quickly takes the container to the elevator. It explodes inside the elevator, which is why it's broken in the present.
- Everyone Has Standards: Even the Creepy Crossdresser who used to live in Penny's apartment considered Sheldon "crazy".
- Hoist by His Own Petard: Or as Sheldon puts it "hoisted by my own spam filter". He set Leonard's messages as spam after he forwarded him a picture of a cat play a piano with the title "This is funny". Unfortunately, this prevents Sheldon from seeing the message Leonard sent regarding the couch.
- How We Got Here: Leonard explains how the elevator got broken, how he first met with Sheldon, and why the Roommate Agreement exists.
- I Owe You My Life: Leonard gives this as one of the reasons he puts up with Sheldon's nonsense, as he feels indebted for saving him from the rocket fuel explosion.
- I Warned You: When Sheldon warns Leonard he hadn't calibrated his rocket fuel for a model rocket properly, Leonard dismissed him, claiming when it came to applied physics, he knew what he was doing...only to learn Sheldon was right when the fuel container started steaming.
- Mathematician's Answer: When Sheldon comes home to find that Leonard had bought a couch and invited Howard and Raj to play games, the following exchange occurs.Sheldon: What are you sitting on?
Howard: I can't speak for these guys, but I'm sitting on my tushie.
- Russian Reversal: During their argument, Leonard yells "Screw the roommate agreement!". Sheldon retaliates "No! The roommate agreement screws you!"
- This Is Going to Be Huge: When making the Roommate Agreement, Sheldon adds a rule to reserve Friday nights for watching Firefly under the assumption that it was going to run for years.
- Whole Episode Flashback: The majority of the episode takes place back when Leonard first moved into the apartment.